In recent years, organic semiconductor materials have been found to be superior to inorganic semiconductor materials in electrical characteristics, and have been increasingly developed and applied to various fields of electronic devices. An organic semiconductor device such as an organic TFT (thin film transistor) includes an organic semiconductor thin film as a semiconductor channel. The organic semiconductor device can easily be processed compared to an inorganic semiconductor device, and therefore can be obtained by a simple low-cost manufacturing process. Moreover, since the organic semiconductor device can be manufactured at about room temperature, a semiconductor technology using a plastic substrate becomes available. Thus, the organic semiconductor device is expected as a post-silicon semiconductor.
Various methods have been studied to manufacture a crystalline organic semiconductor thin film used in the organic TFT in view of the material characteristics. Conventional methods include, e.g., a vapor deposition method, a molecular beam epitaxial method, a solvent evaporation method, a melt method, and a Langmuir-Blodgett method. Among these methods, the solvent evaporation method is simple and able to provide a high-performance organic semiconductor thin film. In particular, the process of the solvent evaporation method is based on an application method of a solution such as droplet formation, spin coating, or printing. Therefore, the solvent evaporation method shows great promise to manufacture an organic semiconductor thin film at about room temperature in a simple and inexpensive manner.
In the application method, a solution of an organic semiconductor material is applied or dropped to the surface of a substrate, and then a solvent contained in the solution is dried. As the solvent is evaporated, the solution becomes saturated and crystals are precipitated from the solution. Thus, an organic semiconductor thin film is formed. Known technologies such as droplet formation and spin coating can easily be applied to the manufacture of a large-area organic semiconductor thin film. The performance of the typical organic TFT formed by the application method is represented by a carrier mobility as high as about 0.1 cm2/Vs. However, such a value for the conventional mobility is unsatisfactory. The mobility is insufficient because the presence of the grain boundary and the irregularity of the molecular orientation interfere with the charge transport.
In order to improve the regularity of the molecular arrangement, the growth of an organic semiconductor single crystal thin film has been studied. For example, Patent Document 1 discloses an improved method for manufacturing an organic semiconductor single crystal thin film having electrical characteristics, particularly high charge mobility, required for the organic TFT by using the solvent evaporation method based on droplet formation.
The manufacturing method of Patent Document 1 will be described with reference to FIGS. 12 to 15. FIG. 12 is a perspective view showing a basic process. FIG. 13 is a cross-sectional view of FIG. 12. The manufacturing method uses a substrate 20 and an edge contact member 21 made of a resin. A raw material solution containing an organic semiconductor material and a solvent is supplied to the substrate 20 and brought into contact with the edge contact member 21 to form a droplet 22, as shown in FIGS. 12 and 13. In this state, the droplet 22 is dried, and an organic semiconductor thin film 23 is formed on the substrate 20. The edge contact member 21 includes a planar contact face 21a as a part of the end faces that intersect the surface of the substrate 20. The droplet 22 is formed in contact with the contact face 21a. 
In the manufacturing process, first, the edge contact member 21 is placed on the substrate 20 so that the contact face 21a is perpendicular to a predetermined direction A of the substrate 20. In this state, when the raw material solution is supplied, the droplet 22 of the raw material solution is held by the contact face 21a under the action of a constant force. The solvent contained in the droplet 22 is evaporated by a drying process while the droplet 22 is held by the contact face 21a. Then, as shown in FIG. 13, the saturation of the raw material solution and the precipitation of crystals of the organic semiconductor material occur successively in the far end portion of the droplet 22 relative to the contact face 21a in the direction A. In FIG. 13, alternate long and short dash lines e1, e2 indicate the movement of the far end of the droplet 22 due to the evaporation of the solvent. As the solvent is evaporated, the crystals grow along the direction A of the substrate 20 toward the contact face 21a, and thus the organic semiconductor thin film 23 is gradually formed.
In the drying process, the attachment of the droplet 22 of the raw material solution to the contact face 21a produces the effect of defining the direction of the crystal growth via the contact with the contact face 21a. This effect can control the crystallinity and improve the regularity of the molecular arrangement of the organic semiconductor material, thereby contributing to an increase in electron conductivity (mobility).
However, in the above basic process, the crystal growth ends with the evaporation of the solvent. Therefore, the size of the organic semiconductor thin film 23 is small, which is less than 1 mm at most. For this reason, to provide a practical organic TFT using the above method, an organic semiconductor thin film array is manufactured, as shown in FIGS. 14 and 15. In FIG. 14, this method uses a contact member 26 including an auxiliary substrate 24 and a plurality of contact protrusions 25 arranged on the auxiliary substrate 24. Each of the contact protrusions 25 has the same function as that of the edge contact member 21 shown in FIG. 12. Specifically, each of the contact protrusions 25 includes a contact face 25a as a part of the end faces that intersect the surface of the auxiliary substrate 24.
First, to create a state in which droplets are held, as shown in FIG. 14, the contact member 26 is located above the substrate 20 with the contact protrusions 25 facing the substrate 20, and the contact protrusions 25 are put on the substrate 20. In this state, a raw material solution is supplied and brought into contact with each of the contact faces 25a to form droplets 22. The droplets 22 of the raw material solution are held by the contact faces 25a, respectively. Then, a drying process is performed to evaporate the solvent contained in the droplets 22. Like the above manufacturing method, crystals of the organic semiconductor material grow as the solvent is evaporated in each of the droplets 22. Consequently, as shown in FIG. 15, organic semiconductor thin films 23a are formed in the positions of the substrate 20 that correspond to each of the contact faces 25a. Thus, an organic semiconductor thin film array can be manufactured.